1. Field of the Invention
The present invention relates generally to telecommunications and more specifically to a filter that suppresses echo in telephonic communications.
2. Related Art
Echo is a phenomenon that occurs when people speak into telephone transmitters and subsequently hear their own voices repeated through the receivers. Simply put, echo is sound energy that is transmitted across the telephone network via a first transmission path, known as a send path, and reflected back to the sender's point of origin via a second transmission path, known as a receive path.
The primary cause of echo is due to the use of hybrids. Hybrids are devices used to interface two-wire telephone circuits to four-wire circuits, while maintaining full-duplex operation in the two-wire circuits. Full-duplex refers to communication circuits that are capable of transmitting and receiving signals in both directions at the same time. In contrast, half-duplex or simplex circuits only permit the transmission of signals in one direction at a time. For economical reasons, two-wire circuits are primarily used to implement the millions of subscriber local loops in commercial public telephone systems. Without the function that hybrids provide, it would not be possible to achieve full-duplex communications using economical two-wire circuits. More typical communication circuits consisting of four wires (one pair used for sending and the other pair for receiving), are used to implement most of the other transmission lines that comprise commercial telephone networks.
The unpleasant effects of echo are minimal in short transmission lines because echo is heard almost simultaneously with the originating speech. In this case, echo is perceived merely as voice feedback emanating from the telephone receiver as the caller talks.
However, in long transmission lines, the echo delay increases and it quickly becomes noticeable and, thus, substantially interferes with telephone conversations. Not surprisingly, telephone engineers have long sought to reduce and/or eliminate signal echo from telephone transmission circuits. Several methods have been used by the telecommunications industry in addressing this problem. Such methods are listed below and are discussed in greater detail subsequently herein.
In order to describe the present invention and the conventional techniques used to control echo, the following terminology and conventions are used throughout this disclosure. In the examples, the two parties having a telephone conversation are referred to as the "calling" party and the "called" party. Generally the calling party is speaking and the called party is listening. The two distinct transmission paths between the calling party and the called party are defined from the point of view of the calling party. Thus, while the calling party is speaking, the voice signal travels along the first transmission path referred to herein as the "send" path. The voice signal is referred to as the "desired" signal, which is to be contrasted with the undesired signal or echo. The echo, which is generated by a reflection of the desired signal from a point close in proximity to the called party, travels along a second transmission path referred to herein as the "receive" path.
Of course during a typical conversation, information flows in both directions. Thus when the called party speaks, the desired signal now travels along the opposite path, described above as the receive path. Likewise, the echo is reflected back to the called party, along the path described above as the send path. During an average two-way conversation, the desired signal and the echo signal continuously change paths and direction, depending on which party is speaking. Consequently, even though only one direction may be discussed in the examples below for the sake of clarity, similar results would occur if the direction of speech and transmission paths were reversed.
Early forms of echo control are quite simplistic in nature. Path attenuation is introduced in both the send path and the receive paths. Since the echo signal must travel twice as far as the desired signal, it receives a greater attenuation and is thereby reduced to acceptable levels. The problem with this method is that the injected attenuation also reduces the energy level of the desired signal. Once transmission lines exceed a certain length, this solution becomes inadequate because the attenuation required to reduce echo to acceptable levels, also degrades the desired signal to unacceptable levels.
An echo control method used in the telecommunications industry is called echo cancellation. This method operates upon a copy of the desired signal from the send path to produce a cancellation signal. The cancellation signal is 180 degrees out of phase with the desired signal. It is then added to the signal in the receive path (that contains the echo), along with the proper delay and attenuation to cancel the echo components therein. The problem with this method is that two devices are needed to effectively cancel echo on both ends of a connection. Since both ends of a connection are often not controlled by the same telephone carrier, the necessary device may be absent on one end or the other, leaving one of the parties exposed to the echo.
The foregoing problem is solved in an echo control device disclosed by the present inventor in U.S. patent application No. 08/580,714, filed Dec. 29, 1995. In that echo control device, a pair of complementary multiple bandpass filters is used to control the echo on both ends of a transmission line from a single point. A voice signal passes through a first multiple bandpass filter that removes a first set of frequency components therefrom. After such components are filtered out by the first multiple bandpass filter, there remains sufficient information in the filtered signal so that voice communication may continue. The echo signal is generated from the voice signal and therefore consists of only a second set of frequency components. The echo passes through a second multiple bandpass filter that filters out the second set of frequency components, thus completely eliminating the echo signal.
While the complementary multiple bandpass filters have been shown to be effective in eliminating echo, the filtering of the calling party voice signals impairs the quality of the voice heard by the called party. The removal of a first set of frequencies from the voice signals renders a low quality voice signal with a hollow sound that may be considered objectionable. Thus, experiments have continued for a set of filter functions that may be applied to the complementary multiple bandpass filters while not degrading signal quality.